Programmable Unijunction Transistor Flasher

This is simple circuit that illustrates the function of the programmable unijunction transistor. It may be quickly wired on a proto-board.

PUT Flasher Specifications

Flash rate: approx 1hZ

Load current: approx 300uA

Minimum voltage: approx 4V

PUT Flasher Circuit Schematic

How it works

The programmable unijunction transistor remains dormant until the voltage across C1 exceeds the gate voltage of Q1 by one diode drop (0.6V) or in this circuit about 6.8V in reference to circuit common. At this point, current starts to flow into the anode of Q1. When the current exceeds the “peak current” threshold (about 1.25uA), the transistor triggers and shorts all three terminals together until the anode current drops below the “valley current” (about 100uA in this case), and the transistor resets itself. When the transistor triggers, it dumps capacitor C1 across the LED. Peak current is limited by the LED internal resistance.

By varying (programming) the gate voltage, the voltage at which the transistor fires varies—in this case, it varies the flash rate.

Background

This cute little device was developed by General Electric about 50years ago. The original GE part numbers were D13K1 and D13K2 (now 2N6027 and 2N6028). The 2N6028 has tighter specifications for more precision applications. On Semiconductor is now the sole manufacturer, but NTE has a relatively expensive equivalent device.

PUT Applications

There are many, many applications for this device, but it is mainly useful for triggering thyristors. Other than for thyristor triggering, the ubiquitous 555 timer has taken over.

Thanks for this. I was working through the Make: Electronics book and it wasn’t working for me. The values were a bit different and that helped me to find my mistake. I had substituted a 470 resistor for a 470k on the anode. I went back and changed it out on their circuit and it also began to flash. Again: THANKS!